1. Field of the Invention
The present invention generally relates to a method and apparatus for measuring the composition of a multiphase fluid, and more particularly to a method and apparatus for measuring the composition of a multiphase fluid using a radiation source.
2. Description of the Related Art
In industrial applications that involve flowing fluids, such as slurries, liquids, chemical, paper, pulp, petroleum, gas, pharmaceutical, food, mining, minerals and vapors and gasses in refinery, it is sometimes beneficial to know certain characteristics of the flowing fluids. For example, in the petroleum industry in which crude oil production is measured each day on its way from the well heads to the refineries, the volumetric flow rate can be an important measurement in process control and optimization.
Several techniques exist for measuring the composition of multiphase fluids by radiating a photon beam through a cross-section of the fluid. Typically, the photon beam is generated by an X-ray tube or chemical radioactive source. These techniques take into account that the absorption of photon beam radiation in a material (including fluids) can be expressed by the following formula:I=Ioe−μx 
where Io is the intensity of the generated radiation (i.e., the intensity of the radiation generated by the radiation source), I is the intensity of the transmitted radiation (i.e., the intensity of the radiation detected by the detector after the radiation has passed through the material), μ is the linear attenuation coefficient of the material, and x is the transmission length of the radiation through the material (which can be a multiphase fluid).
These techniques also take into account that at a specific radiation energy level each material has a specific linear attenuation coefficient and that the linear attenuation coefficient varies in a predictable manner for the material when the radiation level is varied.
International Patent Application No. PCT/EP 94/01320 (published as WO 94/25859) discloses a method for measuring the composition of a multiphase fluid by transmitting a photon beam through the multiphase fluid and detecting the level of radiation absorption at two radiation energy levels. Using the detected levels of radiation (i.e., I) at two energy levels in combination with the assumption that the sum of the three phase fractions in the multiphase fluid, namely, oil, water and natural gas equals to one, produces a set of three linear equations with three unknowns (i.e., the phase fractions of oil, the phase fraction of natural gas, and the phase fraction of water). The set of linear equations is then solved using known techniques to obtain the fluid composition (i.e., the phase fractions of oil, the phase fraction of natural gas, and the phase fraction of water).
European Patent No. 0 236 623 discloses a method for measuring the composition of a multiphase fluid by transmitting a photon beam through the multiphase fluid and detecting the level of radiation absorption at three radiation energy levels. In addition, European Patent No. 0 236 623 discloses a method for calculating the multiphase fluid composition that takes into account radiation absorption caused by sand particles entrained in the multiphase fluid mixture and the sulfur content of the crude oil produced. Information related to radiation absorption caused by the sand and/or sulfur content is obtained using additional radiation systems, which radiate photon beams at energy levels distinct from the energy levels used to determine radiation absorption for the natural gas, water, and oil phase fractions in the multiphase fluid.
European Patent No. 0 896 666 discloses a method for measuring the composition of a multiphase fluid by transmitting a photon beam through the multiphase fluid and detecting the level of radiation absorption at three radiation energy levels. The method disclosed in European Patent No. 0 896 666 takes into account the effect of the salt content in the water fraction on the transmitted radiation (i.e., I).